Flat sheet membranes, films or webs can be manufactured by a variety of methods. Hereinafter, membrane, film, and web may be used interchangeably. Regardless of the method employed to manufacture a film, the final product is typically collected by winding up the film in roll form with the film wound around a central core. The roll of film can be trimmed or slit to a desired width and length. It is highly desirable that the film have a uniform width and, when unrolled, have a minimum amount of camber in order to meet camber specifications defined by the end user.
Camber refers to the curvature along the lateral edges of an elongated film. Camber inherently arises as a result of the film manufacturing process. Specifically, during manufacture (i.e., after stretching and wind-up), the produced film will have a slight thickness variation, for example, in the cross machine direction. The wound-up film will shrink on the roll after a period of time. The slight thickness variation and the shrinkage produce the undesired camber when the roll is unwound. This camber cannot be measured ‘on-line’ during the manufacture of the film. Camber is more evident in wider films, 4 inches or more. Camber is a quality issue in the subsequent use of the film in, for example, the manufacture of batteries, e.g., larger format batteries (those used in, for example, tablets, laptops and hybrid or electric vehicles).
Camber has been historically measured by quantifying the curvature down the lateral edge of a section of the membrane as the amount of deviation in the middle of the sample from a straight line drawn between both ends of the test sample. For example, see FIG. 1. FIG. 1 is a plan view (from above) of sample S secured (via, for example, weight W or tape, not shown) at one end of the sample S to a flat surface (or table), not shown. The sample is flattened on the surface by, for example, de-airing the space between the sample and the surface (e.g., by use of a squeegee). A straight line SL (defined, for example, by a taut string) is placed adjacent one lateral edge of the film. The length X of the sample may be arbitrarily set at any distance, but lengths of 1 or 2 meters (or longer) are common. The amount of camber is measured (for example by a steel ruler or caliper) as the amount of deviation D from the straight line SL at the mid-point C of the sample S.
This camber measurement method is a slow and labor intensive process and is subject to testing variation since human judgment of the tester is involved in the camber measurement process. Current manual camber testing methods or devices in the marketplace are typically accurate only to the nearest 0.5 mm.
A need exists for a practical camber testing method and system that can not only achieve a level of accuracy better than +/−0.5 mm, but that is economical to operate and affordable to purchase, and/or has a level of accuracy preferably to the nearest 0.3 mm, more preferably to the nearest 0.2 mm, and most preferably to the nearest 0.1 mm or better.